This invention relates to an adjustable radial-flow diffuser for a radial-flow compressor which includes adjusting elements with blades. The adjusting elements are arranged in a diffuser annulus and are uniformly distributed along the diffuser circumference with the angular pitch .delta.. For changing the narrow cross-section Q between the blades, the adjusting elements in each case are swivellable about an adjusting axis V which is parallel to the longitudinal axis L of the compressor.
It is an object of a radial-flow diffuser of this type to reduce the flow rate supplied to the gas flow via the compressor rotor and, as a result, to convert its kinematic energy into static pressure. Since at different rotational speeds compressors furnish a very different pressure build-up, the volume flow at the wheel outlet changes differently than at the wheel inlet. It is therefore necessary to adapt the diffusers to the different volume flows in order to achieve, under all operating conditions, an optimal efficiency of the energy conversion. In the case of a radial-flow compressor, the pressure throughput characteristic is a function of the adaptation of the diffuser capacity, which is determined by the narrow area of the diffuser, to the volume flow furnished by the rotor. These narrow diffuser areas are defined by the smallest area which is normal to the flow direction between two adjacent diffuser blades respectively. In the case of a low pressure ratio (in the case of a low rotational speed), the narrow areas of the diffuser are to be as large as possible and, when the pressure ratio is maximal, they should be correspondingly smaller. For this purpose, it is known to design the diffuser blades so that they can be adjusted in different manners in order to be able to vary the mentioned diffuser areas.
For this purpose, it is known to arrange blades on a swivellable rotary table, as disclosed, for example, in Swiss Patent Document CH-PS 492 130. A disadvantage with respect to the efficiency of the diffuser are the gaps between the blades and the areas of the walls outside the rotary plate through which leakage flows flow from the delivery side to the suction side.
An alternative measure for the adaptation of the narrow diffuser area is disclosed, for example, in the European Patent Document EP 134 748 B1. In this case, the narrow cross-section is varied by means of an adjustable diffuser wall. Here, the rigid blades are fitted through slots of the adjustable diffuser wall. However, the step which is formed in the annulus between the diffuser and the rotor causes efficiency-reducing turbulences of the flow. Also in this case, the area ratio which changes during the adjusting in the bladeless annulus between the rotor and the diffuser is a disadvantage.
Based on the above, there is therefore needed a radial-flow diffuser according to the above-mentioned type provided without any blade gap which largely avoids efficiency-reducing steps in the annulus.
According to the present invention, this need is met in that the adjusting elements have a U-shaped or an H-shaped cross-section, the sector-shaped plates mounted on both sides on the blade roots forming the legs and the diffuser blade forming the web.
The development according to the invention has the advantage that blade gaps are avoided between the blade delivery side and the blade suction side where there is a high pressure difference. Because of the slight pressure difference in the circumferential direction, a damaging flow on the rear side of the plates is insignificant. It is advantageous with respect to the efficiency for the blades to be able to be swivelled as a whole, whereby the area ratio and the length/width ratio of the diffuser ducts may be the optimal area. In particular, in the inlet area, the radial-flow diffuser is free of steps which are disposed transversely with respect to the main flow direction, whereby additional aerodynamic losses are eliminated.
In order to avoid losses as a result of incorrect positioning of the adjusting elements, the adjusting elements can preferably be swivelled synchronously and in the same direction by means of an adjusting mechanism.
For an aerodynamically favorably constructed radial-flow diffuser, it is important that transitions from the rotor to the diffuser annulus are constructed to be as continuous as possible along a large adjusting range of the adjusting elements.
A simple bearing of the adjusting element in the diffuser housing by way of bearing journals which are concentric with respect to the adjusting axes ensures an exact adjustability also in the case of thermal stress.
The clearance between the plate sides of adjacent adjusting elements which opens or closes during the adjusting movement and extends in the main flow direction is optimized in a fluidically advantageous manner. As a result, the dBe width of the clearance along the length of adjacent plate sides always remains constant. An aerodynamically unfavorable tilting of the clearance with respect to the flow direction may therefore be avoided along the whole adjusting area.
Another improved development of the invention is provided whereby the diffuser ducts of the throttled radial-flow diffuser, that is, in the case of the position of the adjusting elements which pertains to the smallest narrow cross-section, remain without clearances between the plates and without its influence on the flow. In the case of an unthrottled position of the adjusting elements, that is, in the case of a maximal size of the narrow cross-section, the plate outlet edges are disposed on a joint circle so that a continuous diffuser outlet is formed.
In the case of a further development of the invention, it is ensured that the adjusting elements within their adjusting range can be swivelled between the plates without the opening of a clearance. As a result, an improvement of the flow quality in the diffuser ducts can be achieved, particularly in the case of an unthrottled operation of the diffuser.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.